It is well recognised that high in performance photovoltaic cells there is a need to avoid heavily diffused surfaces where the light enters the cell, and preferably sheet resistivities above 100 ohms per square should be used in conjunction with good surface passivation. Conventional screen-printed solar cells cannot satisfy this requirement due to the need for the metal fingers to be spaced well apart and due to the need for heavy doping in the silicon to achieve good ohmic contact between the silicon and the metal. Both factors require emitter sheet resistivities well below the desired 100 ohms per square, with corresponding degraded electrical performance.
Referring to FIG. 1 a conventional screen-printed solar cell is illustrated in which the base region 11 of the silicon wafer is doped p-type and a heavily diffused n++ emitter 12 is used to provide good conductivity in the surface region. Wide metal fingers 13 are formed over the surface to form the front surface metallisation, the fingers being spaced well apart and having a large metal/silicon interface area. A heavily doped p-type region 14 is provided at the rear of the base region 11 to provide a good interface with the rear contact metallisation 15. Screen-printed solar cell technology dominates commercial photovoltaic manufacturing, with well over 50% share of international markets. Despite the dominance of this technology, this solar cell design shown in FIG. 1, has significance performance limitations that limit the cell efficiencies to well below those achievable in research laboratories around the world. In particular, the front surface screen-printed metallisation 13 necessitates a heavily diffused emitter 12 to achieve low contact resistance and also to achieve adequate lateral conductivity in the emitter since the metal lines need to be widely spaced compared to laboratory cells to avoid excessive shading losses. Such cells therefore typically have emitters with sheet resistivities in the range of 40-50 ohms per square, which inevitably give significantly degraded response to short wavelength light. To raise this sheet resistivity to above 100 ohms per square as required for near unity internal quantum efficiencies for short wavelength light, serious resistive losses are introduced, both in the emitter 12 and by way of the contact resistance at the metal to n-type silicon interface.
To address this issue, researchers are endeavouring to develop improved screen-printing techniques that facilitate the achievement of narrower screen-printed metal lines (fingers) that can therefore be spaced more closely without excessive metal shading losses, while still allowing the use of emitter sheet resistivities approaching 100 ohms per square. An alternative approach which has been considered is to improve the lateral conductivity of the 100 ohms per square emitter, by coating it with a transparent conducting oxide such as zinc oxide or indium tin oxide. This approach gets used in some solar cell technologies such as amorphous silicon solar cells where the lateral conductivity is not high enough for the metal finger spacing being used. Such layers however tend to be expensive to produce reliably in production while simultaneously lowering the durability of the resulting product due to degradation in the electrical properties of such layers over long periods of time, particularly in the presence of moisture.
Another issue with the conventional design of FIG. 1 is that it has quite poor surface passivation in both the metallised and non-metallised regions. Even if good ohmic contacts could be made to more lightly doped emitters closer to 100 ohms per square, the large metal/silicon interface area would significantly limit the voltages achievable due to the high levels of recombination in these regions and hence contribution to the device dark saturation current. These voltage limitations have not been of major significance in the past due to the limitations imposed by the substrates. However, in the future as wafer thicknesses are reduced to improve the device economics, the cells will have the potential for improved open circuit voltages, but only provided the surfaces, including those under the metal, are sufficiently well passivated.